In the field of optical communications, wavelength division multiplexing (WDM) is employed as a solution of transmitting a large amount of data, in which optical carrier signals of multiple wavelengths are multiplexed in and transmitted by an optical fiber. The 100 Gbps Ethernet (100 GE) of fiber-optic telecommunication standard uses four-wavelength WDM that transmits optical signals with a data rate of 4×25.8 Gbps. In recent years, standardization for transmission capacity over 100 GE has been in progress. One example is 400 Gbps Ethernet (400 GE), which uses a 4-level pulse amplitude modulation (PAM-4) WDM that transmits optical signals using four wavelengths with a data rate of 4×56 Gbaud.
Downsizing and energy saving of WDM optical transceivers are also in progress. For a transmitter optical sub-assembly (TOSA) mounted in a WDM optical transceiver, a technology for densely mounting a plurality of laser diodes (LDs) and collimating lenses, and a technology for downsizing an optical multiplexer/demultiplexer, and other technologies are being developed. With the rise of silicon photonics in recent years, further downsizing and cost reduction are being achieved.
In silicon photonics, the components other than LD, such as an optical modulator, a modulator-driver integrated circuit (IC), optical multiplexers/demultiplexers, photodiodes (PD) for monitoring the output power, etc. are fabricated in a silicon substrate, and the LD is hybrid-integrated onto the substrate. Because an LD cannot be fabricated of an indirect bandgap material such as silicon, it is made of a compound semiconductor with a direct bandgap and mounted on a silicon photonics substrate.
Some multi-wavelength lasers have been proposed for WDM telecommunications. See, for example, Patent Documents 1 and 2 listed below. There is a demand for a multi-wavelength light source with less power consumption, while preventing the throughput yield from decreasing.